5 Frequencies of chromosome abnormalities A chromosome abnormality is present in 40-50% of all recognized first-trimester pregnancy loss. Approximately 1 in 6 of all pregnancies results in spontaneous miscarriage. Birth prevalence of chromosome abnormalities is 0.5-1%.

7 Trisomy Presence of an extra chromosome, the total number of chromosomes is 47 in a somatic cell. Trisomy usually results from meiotic nondisjunction. There are 3 of number 21 chromosomes

8 Monosomy Absence of one chromosome, so the total number of chromosomes is 45 chromosomes in a somatic cell. Usually only seen as 45,X, (autosomal monosomy is usually lethal). Monosomy usually results from meiotic non-disjunction.

9 There are 2 types of cells in an individual, for example normal 46,XY cells and abnormal trisomic cell line 47,XY,+21. The two cell lines are derived from the same zygote due to mitotic nondisjunction. Mosaicism

10 Triploidy Presence of 3 haploid sets : 23 x 3= 69 chromosomes (haploid set =23, diploid set =46). Usually incompatible with life and seen only in abortions. May results from 2 sperms fertilising the ovum or retainment of the polar body with the ovum.

11 Normal meiosis, the 46 chromosomes become 23 in each gamete

12 Non-disjunction during meiosis means that one daughter cell gets 24 chromosomes and the other 22 chromosomes

13 Fertilisation of the gamete carrying 24 chromosomes (extra number 21 ) with a normal gamete having 23 chromosomes results in a zygote of 47 chromosomes (trisomy 21 zygote)

15 Robertsonian translocation occurs between 2 acrocentric chromosomes with breaks near centromeres and union of the long arms

16 Carriers of balanced translocations are healthy but They are at risk of having offspring with unbalanced chromosome constitution This may present as: Repeated spontaneous abortions Stillbirths Birth defects Intellectual disability

17 Deletion: loss of part of a chromosome

18 Pericentric inversion: two breaks with inversion of the segment in between

23 Down syndrome (DS) The overall prevalence at birth is approximately 1 in 650 to 1 in 700 births. May be higher in some countries where women continue to bear children at an advanced age.

24 Clinical features of DS The most common finding in the newborn period is severe hypotonia. Single palmar creases are found in 50% of Down syndrome children in contrast to 2-3% of the general population. Congenital cardiac abnormalities are present in 40-45% of babies with Down syndrome. Hypothyroidism.

26 Natural history of DS Affected children show a broad range of intellectual disability with IQ scores ranging from 25 to 75. The average IQ of young adults with Down syndrome is around 40 to 45. Social skills are relatively well advanced and most children with Down syndrome are happy and very affectionate.

27 Natural history of DS Adult height is usually around 150cm. In the absence of a severe cardiac anomaly, which leads to early death in 15-20% of cases, average life expectancy is years. Most affected adults develop Alzheimer disease in later life due to dosage effect of the amyloid precursor protein gene.

28 Chromosome abnormalities in Down syndrome 95% of cases are trisomy 21, 47,XX,+21 (47,XY,+21), risk of having trisomy 21 increases with advanced maternal age. 4% are due to translocation between chromosome 21 and another acrocentric with a total number of chromosomes =46, but the genetic material of chromosome 21 is present in triplicate. The translocated chromosome is usually inherited from a normal carrier parent. Such a translocation carrier parent has a risk of having a Down syndrome with each pregnancy (about 20% if mother is carrier and 5% if father is carrier). 1% mosaic cases ( 46,XY/47,XY,+21).

29 Salihu, 2003, Obstet Gynecol

30 Meiosis in Robertsonian translocation carrier who has 45 chromosomes ( only one 21) but the other chromosome 21 is translocated to chromosome 22 = normal amount of genetic material

31 The gamete carrying 23 chromosomes but one is a translocation 21/22 is fertilised by a normal gamete resulting in a zygote with translocation Down syndrome

32 Turner syndrome: monosomy X The two main medical problems are short stature and ovarian failure. Ovarian failure leads to primary amenorrhea and infertility. Estrogen replacement therapy should be initiated at adolescence for the development of secondary sexual characteristics and long-term prevention of osteoporosis.

36 Turner syndrome is being detected early in pregnancy as a result of routine detailed ultrasound scanning, which can reveal either generalized edema (hydrops) or swelling localized to the neck (nuchal cyst or thickened nuchal pad).

40 XYY MALES Fertility is normal. Physical appearance is normal and stature is usually above average. Intelligence is mildly impaired, with an overall IQ score of points below a control sample. The additional Y chromosome must arise as a result of non-disjunction in paternal meiosis II or as a post-zygotic event.

41 XXX females These women usually have no physical abnormalities but can show a mild reduction of between 10 and 20 points in intellectual skills below their siblings. This is rarely of sufficient severity to require special education. Women with a 47,XXX karyotype usually show normal fertility and have children with normal karyotypes.

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